1. Field of the Invention
This invention relates to fluid pressure operated impact mechanisms of the type having reciprocating members moved by introducing a pressure medium into a pressure chamber at one side of the reciprocating member, and relates more particularly to sealing arrangements for maintaining the pressure chamber in a sealed condition during motion of the reciprocating member. Impact mechanisms of the reciprocating-motion, fluid pressure operated type to which this invention relates include devices for pile and sheet pile driving, for drilling, chiseling, marking and hammering, for vibrating screens, feeder pans, and similar devices, and for compacting ballast, or the like.
2. Description of the Prior Art
A variety of reciprocating-motion, fluid pressure operated devices are known. A typical example is a fluid pressure operated hammer used for pile driving and made in the form of a piston-and-cylinder device, the impact force being produced by lifting the piston with a pressure medium acting on its bottom surface, and then dropping the piston against a stop plate, which may comprise one of the cylinder end walls. To accomplish such lifting and dropping, an impulse generating and valving system is required for alternate delivery and discharge of the pressure medium to a chamber under the piston. Further, to avoid energy losses, good sealing is required between the piston and cylinder. These features require parts manufactured with considerable precision, which will be expensive. Due to such demands for precision, it is particularly expensive to make a piston-and-cylinder device with a large diameter in order to obtain a large pressure area, and large lifting force, for the piston. Piston-and-cylinder devices also face problems because of the interaction between the piston guiding means and the pressure medium. The pressure medium tends to expel lubricant from between the piston and cylinder wall and, in addition, contaminants in the pressure medium can damage the guiding surfaces along the cylinder walls.
As a result of these considerations, present reciprocating motion, fluid pressure operated devices of this type are not entirely satisfactory.